As society becomes increasingly reliant upon technology, mechanical and electromechanical systems, such as aircraft, automobiles, weapons systems and power systems, are called upon to perform an ever increasing number of functions. One downside to this is that, in some situations, a failure of a single threaded component in the system may cause a catastrophic failure of the entire system possibly resulting in the loss of millions of dollars and hundreds of lives. In an attempt to reduce the probability of a catastrophic systems failure, critical and some non-critical systems are required to satisfy predetermine operating tolerances before they may be used. As such, key threaded components within these systems, i.e. threaded components whose failure may cause a catastrophic system failure such as screws and/or gages, must also satisfy operating tolerances. If a threaded component fails to satisfy these required design tolerances and/or performance specifications, a degradation of system performance and/or a total system failure may occur resulting in damage to the system and/or injury/loss of life to an operator.
One of the current systems used for inspecting the physical characteristics of a threaded component employ an attribute inspection approach that measures the characteristics of the threaded component via a contact measurement technique which does not protect product design limits. This technique uses GO and/or No Go ring gages that are adjusted, or calibrated, to a desired thread measurement via Go and/or No Go setting plugs. Unfortunately, this technique does not ensure the integrity of design limits and because this approach is dependent upon human interaction, this technique has the disadvantage of being time consuming, subjectively inaccurate and unreliably repeatable for tight operating tolerances, thus permitting threaded components having dimensionally non-conforming characteristics to pass inspections. Moreover, there is a considerable wear factor on the measuring instruments, requiring the Go, No Go setting plugs to be inspected and replaced often.
Another approach used for measuring external thread gages utilizes three wires communicated to the gage being measured. The three wires are of a known diameter and are typically disposed between the threads of a component such that the wires protrude from the threads, wherein two wires are disposed on one side of the threaded component and one wire is disposed on the opposing side of the threaded component. The diameter over the wires is then measured via a human inspector. Because the wires are of a known diameter, this allows certain characteristics of the threads to be determined by measuring the width of the wires disposed between the threads. Unfortunately, this approach is also dependent upon human interaction. If the inspector measuring the distance over the wires compresses the wires too much, the wires may become deformed resulting in an inaccurate measurement. Additionally, the surface finish of a threaded component may adversely affect the accuracies of these measurements. Moreover, because the wires are loose and are not held between the threads, the wires may be dropped which may result in the wires becoming contaminated with dirt, the wires being lost or, if someone steps on them, the wires being deformed.
Furthermore, different operators will generate different gage pressures on the wires which may cause erroneous readings. Thus, this approach has the similar disadvantage of being time consuming, subjectively inaccurate and unreliably repeatable for tight operating tolerances, thus also permitting threaded components having dimensionally non-conforming characteristics to pass inspections. Additionally, the reliability and repeatability of this measurement is very poor because an operator must measure angles using an optical projection which is also time consuming, inaccurate and often fails to satisfy current product and gage calibration specifications. As such, the Measurement Uncertainty Factor (MUF) in many situations exceeds the required tolerances and as a result, the required accuracies for complete certification of these methods have thus far been unobtainable.
Another disadvantage to current measuring systems involves the human element required to obtain the actual physical measurement of the component and the data obtained from that measurement. As such, the accuracy of the equipment used to obtain the measurements is very questionable. For example, using current technology and methods, if two different people measure the same characteristics of a single component, it is highly probable that they will obtain different results. Another example involves when the component being measured is not be correctly aligned within the inspection system. This is undesirable because data obtained from the measurement of an incorrectly aligned component will most likely contain errors related to the misaligned component and thus the resultant measurement will be incorrect. Still yet another example involves errors in the functional size of the component due to deviations or waviness in the helical path of the thread. This creates an assembly problem in that parts may not assemble correctly. This is undesirable because data obtained responsive to an incorrect function size is unreliable and may result in a failure of the component or assembly. Another example involves deviations (in lead, flank angle error, pd error, major & minor diameter error, root radius roundness, etc.) from the true line measurements of the component.